Monday, March 24, 2014

Late Winter Rains and Army Cutworms

Southwest Idaho's Winter of Ice Fog ended when snow fell in early February. The ridge of high pressure that had smothered us under a season-long inversion broke up. This allowed a procession of rainstorms to wash in from the Pacific.

The Treasure Valley smelled of damp, warm soil. Ranchers, farmers, and water managers cheered the promise of grass, irrigation water, and ample snow pack. An artist used every shade from Absinthe to Wintergreen to paint the Boise Green Belt in living color.

The rains were too late to save this year’s crop of cheatgrass in the dry areas along the Snake River, south of town. Last fall, a prodigious storm had germinated a flush of the winter annual grass, along with its annual mustard cousins. Sadly for the plants, their good luck didn't last. Happily for me, their misfortune confirmed an accusation I'd made 11 years earlier.

Large numbers of miller moths had preceded the rains. The eggs they laid hatched into army cutworms a month or so later. The larvae soon got down to business eating the tiny green plants.

The dry winter that followed was ideal for the cutworms, which develop fungal diseases in damp weather. But the cheatgrass and mustards struggled without rain. The annual plants died from lack of water or were consumed by army cutworms. Perennial grasses, mostly short Sandberg bluegrass, survived on the hills above areas where the annuals had died.

Hungry army cutworms roamed the bare areas looking for food...

...or hid under cowpies, out of the wind...

...but within reach of hungry centipedes.

The larvae became arboreal and climbed sagebrush...

...and fourwinged saltbush and kept eating.

Army cutworms also climbed the hills to munch on Sandberg bluegrass, which seemed able to outgrow the larvae's feeding.

When they ran out of plants to eat, the cutworms dined on their fallen relatives.

In 2003, I saw bare areas around Winnemucca, NV that resembled this year's dieoffs along the Snake River. After months of sleuthing, I found that army cutworms were probably responsible for the 2003 damage. This year, I caught the culprit cutworms in the act.